With the continued advancement of glass curtain wall technology, the popularity of these systems has skyrocketed to an all-time high.
Punched-window buildings have become passé as more designers choose to wrap their buildings with floor-to-ceiling glass curtain walls fitted with narrow mullions and high-performance glass.
While glass façades offer a wealth of benefits to building occupants—from improved views to abundant daylight—these systems can present Building Teams with internal sound transmission issues, especially in acoustically sensitive environments like music-related facilities.
"We run into this condition quite often," says Ronald Eligator, principal consultant in the New Rochelle, N.Y., office of Acoustic Dimensions, which specializes in acoustic design for performing arts, sports/entertainment, worship, production, and corporate office projects. "I think glass curtain walls are not appropriate for applications where a high degree of sound isolation is required."
Therein lies the problem: More architects are specifying glass curtain wall systems for buildings with sound-sensitive environments, including some mid- and high-rise condo buildings.
"Glass curtain walls are perfectly fine for office buildings, even medical offices where HIPAA needs to be complied with," says Eligator, referring to the medical privacy requirements of the Health Insurance Portability and Accountability Act. "But in more stringent environments, like music schools, it doesn't always work. We're always arguing for a different type of exterior wall construction with punched windows in those applications."
In working with curtain walls, Eligator must develop creative ways to control internal sound transmission that can occur horizontally from room to room and vertically from floor to floor.
One source of sound leakage is the void between the curtain wall and the interior wall partitions. "It's important to tightly seal the partition to the curtain wall, usually to the mullion," says Eligator.
A major source of sound leakage is the void between the curtain wall and the interior wall partitions. Sound transmission can be reduced by constructing a filler partition composed of two pieces of drywall filled with batt insulation. The wall should be sealed with non-shrinking, non-hardening caulk.
lustration: Acoustic Dimensions
The typical approach involves constructing a filler partition composed of two pieces of drywall filled with batt insulation (see illustration). The partition should be thinner than both the interior wall and the curtain wall mullion. The first acoustic compromise encountered is that this thinner wall section reduces the overall sound barrier performance of the main wall partition.
Where the filler partition meets the mullion, Eligator recommends placing a strip of closed-cell neoprene tape to accommodate movement. "Wind forces may cause the curtain wall to move, so you have to be careful about making that connection too rigid," he says.
The connection should then be caulked with a non-shrinking, non-hardening caulk to prevent sound leakage.
Alternatives to this method are proprietary products such as Mullion-Mate. The spring-load device from Gordon Inc., Bossier City, La. (www.gordonceilings.com), snaps into place between the curtain wall mullion and the wall partition, creating a tight seal for gaps as small as two inches and as large as 65/8 inches, depending on the model. The device can be specified with acoustical batt insulation to control sound transmission (tested STC: 38), and can be anodized or painted to match the mullions.
"It's nice in concept, but in application it can be challenging, because the device needs to be plumb and square at each opening," says Eligator, who has worked on a few projects where Mullion-Mate was specified. "We've had to go back and have terminations caulked to ensure a good seal."
Building Teams may also have issues controlling sound transmission through the curtain wall system itself.
Key components such as the curtain wall framing, mullions, and even the glass can serve as a sound transmission path between rooms and floors. These paths are especially critical in acoustically sensitive applications, which may include commercial or residential projects.
"In some cases, no matter how well you seal the wall partitions, sound is going to transmit through the components of the glass curtain wall system," says Eligator. Happily, there are several ways to minimize this "sound flanking."
One approach involves insulating the mullions by filling them with expanding foam, sand, non-shrinking mortar, caulk, or lightweight cement. Make sure the filler is chemically compatible with the type of aluminum used for the mullions, and chose the substance based on the level of sound isolation required, says Eligator.
Some manufacturers offer insulated mullions as an option for thermal insulation, which can be helpful in applications with stringent sound-isolation needs.
Another solution involves installing sound isolation devices within the curtain wall mullion. PAC International, Las Vegas, and Acoustiblok, Tampa, Fla., have teamed to develop a version of PAC's Resilient Sound Isolation Clip specifically for curtain wall mullions (www.pac-intl.com). The RSIC-AMI Window Mullion system is composed of PAC's clips installed within the mullion and surrounded by Acoustiblok's sound isolation material to provide a tested STC rating of 58.
Also, consider specifying a curtain wall system with thicker mullions (the thinner the gauge, the more sound transmission.), and assess whether the mullions run horizontally versus vertically.
"Many manufacturers have systems with continuous vertical mullions that interrupt the horizontal mullions," says Eligator. "You get better sound isolation room to room, but that can make the vertical, floor-to-floor isolation worse."